Malgorzata A. Pozor College of Veterinary Medicine, University of Florida, Gainesville, FL, USA The stallion penis consists of three parts: the root, the body or the shaft, and the glans. The root has two crura, which are attached to the tubera ischii via the ischiocavernous muscles on each side. Suspensory ligaments deliver further support and stabilization to this attachment [1]. The crura fuse below the ischial arch and form the laterally compressed corpus cavernosum. The urethra is surrounded by the corpus spongiosum, which starts at the bulb of the penis and continues all the way to the urethral process. The urethra lies in the urethral groove, on the ventral side of the penile body [1]. The bulbospongiosus muscle runs along the ventral aspect of the penis and encloses the corpus spongiosum of the urethra (Figure 12.1). The corpus cavernosum is enclosed by a thick fibrous sheet termed the tunica albuginea (Figure 12.1), the elastic properties of which allow limited expansion of the penis during erection [2]. Further anteriorly, the corpus cavernosum divides into three processes – one central process and two lateral processes. The central process is the longest and runs all the way into the glans penis. The glans penis is the most distal part of the penis and has two distinct parts – neck and corona glandis. The erectile body of the glans is called the corpus spongiosum glandis (Figure 12.2). The corpus spongiosum glandis has a dorsal process, which covers the dorsal and lateral sides of the corpus cavernosum penis for approximately 10–15 cm (4–6 inches) [3]. The glans can expand significantly during erection due to a profound elasticity of the surrounding tissues. The most distal part of the glans penis has a deep depres- sion, the fossa glandis, which hosts the terminal portion of the urethra, the urethral process. Furthermore, the fossa glandis has a bilocular diverticulum, called the urethral sinus [3]. Smegma may accumulate in the urethral sinus and form thick “beans”, which are often manually removed prior to semen collection (Figure 12.2). Figure 12.1 Diagram of stallion penis and cross-sections through a specimen: A – proximal part of the penile body; B – middle part of the penile body; C – free part of the penile body; D – distal part of the penile body. a: tunica albuginea; b: incomplete septum; c: corpus cavernosum; d: corpus spongiosum and urethra; e: bulbospongiosus muscle; f: retractor penis muscle; g: dorsal artery; h: deep artery; i: dorsal vein; j: corpus spongiosum glandis – dorsal process. Figure 12.2 Diagram of a distal part of stallion penis and cross-sections through a specimen: (A) distal penis; (B) collum glandis; (C) corona glandis; (D) distal part of the glans penis. a: tunica albuginea; b: corpus cavernosum; c: corpus spongiosum and urethra; d: corpus spongiosum glandis; e: retractor penis muscle; f: connective tissue and penile fascias; g: urethra; h: diverticula of urethral sinus; i: fossa glandis; j: urethral process. The main source of blood supply to the stallion penis is the internal pudendal artery, as well as the obturator artery [4]. The internal pudendal artery gives off the artery of the bulb, the deep artery of the penis, and the dorsal artery of the penis. The obturator artery gives off the middle artery of the penis, which anastomoses with the dorsal artery of the penis and with the cranial artery of the penis [5]. The latter comes from the external pudendal artery. Ultrasound evaluation of the stallion penis is not performed routinely. However, this technique is helpful in detecting penile pathologies, mostly associated with paraphimosis, priapism, and penile trauma. Prior to the ultrasound examination of the penis in a normal stallion, sedatives are given to induce the penis to drop from the preputial cavity. Phenothiazine derivatives should be avoided as they can cause paraphimosis. High-frequency (7.5–10 MHz) linear or micro-convex transducers are recommended. Prior to the examination, the penis should be washed with warm water in order to remove smegma, which could interfere with penetration of the ultrasound waves. A copious amount of warm ultrasound gel is then applied to the penis. Once the examination is completed, the remaining gel is washed off with warm water and a cotton towel. The ultrasound architecture of the corpora cavernosa, the corpus spongiosum of the urethra, and the corpus spongiosum of the glans are visualized on the longitudinal and cross-sections of the organ (Figures 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, 12.10, 12.11, 12.12
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Ultrasonography of the Penis
Anatomy
Ultrasound Evaluation of the Stallion Penis
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